This invention relates generally to optical devices and systems and, more particularly, to controlling the thermo-optic behavior of optical paths within such devices and systems.
In general the properties of an optical device are sensitive to changes in its temperature. For example, the output wavelength, the optical power output and current threshold of a semiconductor laser are sensitive to temperature. Similarly, the propagation constants of optical modes in a semiconductor or glass waveguide also change with temperature. Typically optical devices and systems are designed to account for such thermally-induced shifts, at least in those device/system parameters that are the most important for proper performance of the device/system.
Consider, for instance, optical networking that is currently in an exponential growth mode. The design and deployment of new dense wavelength division multiplexed (DWDM) systems is having a profound effect on the availability and use of bandwidth by vastly expanding the amount of information that can be transmitted on a single optical fiber. Because such systems are so new, much of the growth occurring in the last few years, opportunities exist to contribute subsystems that improve or simplify aspects of the operation of a DWDM network. One way to provide such improvement and/or simplification is to be able to control the thermo-optic behavior of the system components over an operating range xcex94T.
In a DWDM system the channel wavelengths are packed together at minimal spacing. Therefore, any significant drift in the output wavelength of the lasers (used as carrier signal sources) is a serious problem. One source of wavelength drift is aging of the lasers. Wavelength changes due to aging can be corrected by changing the temperature of the laser by means of a thermoelectric cooler/heater (TEC). Although the wavelength of each lasers can be controlled by an etalon used as a frequency discriminator, the optical path length of the etalon is also temperature sensitive. Thus, changing the temperature of the laser might also change the temperature of the etalon if the two devices are located in the same package. However, other phenomena can also affect the temperature of the etalon and hence its optical path length; e.g., aging (1) of the temperature control circuit used to maintain the laser at a predetermined temperature and at a predetermined output wavelength; and (2) the temperature gradient produced across the etalon because the TEC is typically located at the base of the etalon.
Consequently, there is a need in the frequency discriminator art for an etalon that is essentially temperature insensitive over the temperature operating range of the corresponding optical system.
There is also a need in optical systems in general to be able to control the thermo-optic behavior of an optical path such as, for example, by compensating or balancing one parameter against another, or by enhancing a selected parameter. The latter finds application in temperature sensors, whereas the former finds application not only in the etalon discussed above, but also in a broad spectrum of optical apparatus ranging from telescopes to Mach-Zehnder interferometers.
In accordance with one aspect of our invention, a method of controlling the thermo-optic behavior of an optical path over a range of temperatures xcex94T comprises the steps of determining a figure-of-merit (FOM) for the optical path and including in the path a body of crystalline material that enables the conditions specified by the FOM to be satisfied. The crystalline material is highly transparent at a wavelength xcex of radiation propagating in the path, and it has a coefficient of thermal expansion (CTE) and a refractive index n such that the CTE and dn/dT are mutually adapted to satisfy the FOM over the range xcex94T. In one embodiment, the CTE and dn/dT of an etalon compensate one another so as to perform frequency discrimination that is essentially temperature insensitive over the range xcex94T. In a preferred embodiment of the optical etalon the crystalline material comprises LiCaAlF6.
In accordance with another aspect of our invention, an article of manufacture (e.g., apparatus, device, subassembly, subsystem, system) comprises an optical path over which radiation at a wavelength xcex propagates, and, disposed in the path, a body of crystalline material that enables the conditions specified by the FOM to be satisfied. The crystalline material is highly transparent at a wavelength xcex of radiation propagating in the path and has a coefficient of thermal expansion (CTE) and refractive index n such that the CTE and dn/dT are mutually adapted to satisfy the FOM over the range xcex94T. In one embodiment, the CTE and dn/dT of an etalon compensate one another so as to form a frequency discriminator that is essential temperature insensitive over the range xcex94T. In a preferred embodiment of the etalon the crystalline material comprises LiCaAlF6.